Energy Harvesting Reaps Rewards for Sensor Networks

Wireless sensors are being used in commercial and domestic buildings, vehicles and industrial settings, although some are also finding themselves deployed in nature to monitor wildlife. As new applications emerge, the role of the sensor will change, as well as the way they are powered.

Energy harvesting is an attractive option to reduce the power consumption and help manage the ongoing labor costs of periodically changing batteries [in wireless sensor networks].”

One of the main challenges for developers is to keep track, not just of where sensors are being used (i.e. locations), but also sensor applications. Wireless sensors are increasingly used in harsh environments and also to add intelligence and comfort to business premises and homes.

Figure 1: Wireless sensors within the home, and commercial buildings, are the cornerstone for ‘smart’ buildings.

Karen Lightman, Executive Director of the MEMS and Sensors Industry Group describes the scope of the industry: “The main challenges are keeping up with the new applications that are using MEMS/sensors to increase user interactivity, gather intelligent data via remote sensing, and actuate. Emerging technologies are numerous, and areas such as autonomous vehicles, with their use of LIDAR, RADAR, and sensing are very exciting. MEMS/sensors are the cornerstone of smart home, smart cities and IoT applications of all flavors,” she adds, with new sensors types based on optical MEMS (micromirrors) and environmental sensors now emerging.

DifferentiationThe evolution has to be matched with software support. “MEMS/sensors suppliers need to keep pace with algorithm development and machine learning in order to process all the data generated, because in many cases, software capabilities still exceed the capabilities of the hardware,” she points out. “Taking full advantage of software algorithms and machine learning will allow MEMS/sensors suppliers to further differentiate their offerings, increasing their competitive position.”

Rather than single entities, sensors are part of a system solution, which brings its own issues. One is standards. “We need standards to help the ecosystem scale, without getting bogged down by single proprietary solutions from individual companies,” urges Lightman.

System integration also means that power consumption is important. Dr Eric Mounier, Senior Technology & Market Analyst, MEMS Devices & Technology, at Yole Développement, sees smart buildings as a promising sector. At present 40 percent of world energy being used is in buildings, and of this, 31 percent is for heating and 26 percent is for lighting. “Smart homes/buildings are definitely something to look at for future smart sensors,” he says. “Smart buildings will require a shift in sensing from basic on/off light switching to more advanced functions, such as dimmer, occupancy control, motionless people detection, people localization, people counting, fire detection, reduced false alarm, and people behavioral analysis,” he predicts. He points out that while commercial buildings have used pyro sensors for many years, they have often been a basic motion detection functionality. However, smart sensors, with more functionality, will be specified.

Jérémie Bouchaud, Director and Senior Principal Analyst, MEMS & Sensors, IHS Markit, agrees. “This is the area where we found the fastest, and most obvious return on investment, by adding more intelligence to the building,” he confirms. “This is driving demand for a variety of sensors such as passive infrared (PIR) sensors, light sensors, thermopile and microbolometer arrays,” he says.

Figure 2: Graph showing the ambient energy sources available from different sources.

Potential for New ApplicationsKathy Vaeth, VP Engineering at microGen Systems, which develops products based on its proprietary Piezoelectric Vibrational Energy Harvester (PZEH) technology, advocates energy harvesting sensors to reduce the power budget. “Energy harvesting is an attractive option to reduce the power consumption and help manage the ongoing labor costs of periodically changing batteries [in wireless sensor networks], as the technology can extend the time between services or eliminate the use of batteries completely,” she points out. “This is particularly important when the sensor modules are installed in remote or hazardous locations, where operator safety can be a concern. In addition to reduced maintenance costs, energy harvesting can optionally open up new applications for wireless sensors installed in extreme environments, such as high temperatures, where battery function tends to fail quickly below 85 to 100°C, depending on the chemistry used.”

EnOcean’s Sales Director, North Europe and the Middle East, John Corbett, agrees: “The maintenance case is growing. Most wireless systems are battery-powered. They will typically last one to two years, particularly in industrial applications, as they are ‘pinged’ a lot more and tend to take more power,” he says. The main benefits can be seen in terms of maintenance costs. “No one wants to replace batteries once they have installed the systems, partly because sometimes wireless sensors are installed in odd places, like on a gear motor or braking system,” he explains. “What tends to happen is that with a high grouping of sensors—say 200 in a small area— if one battery fails, the maintenance team will decide to replace all the batteries, which makes it costly to replace, keep stock and rotate batteries.”

There are three technologies for energy harvesting: kinetic (movement), PV (light) and Peltier/Seebeck effect. The primary one, says Corbett, is kinetic harvesting, using an inductive harvester. This is when the user moves a small element within an inductive coil to power the device. It is typically a small movement, like a switch, or push-button, with a movement of about 0.9mm.

PV cells are used typically for indoor light harvesting, although there are outdoor versions, for example on the roof of a house. The light harvester has a small charge cell to store energy when nothing is happening, says Corbett. Power consumption and longevity depend on what it is asked to do. If it is asked to send a ‘heartbeat’ to reassure the system that it is still active it will not last as long as if it only transmits if it senses any changes. “If you want an ‘I’m alive’ heartbeat every 10 minutes, it can last seven days, if you want a heartbeat every minute, that goes down to three days,” says Corbett.

Heat-powered sensors use a Seebeck or Peltier cell to convert heat into electricity. “If you add power, they go cold on one side and hot on the other, or if you heat them and have a heat sink on the other side, they cool down to a three degree differential, explains Corbett. The postage stamp-sized elements are typically used for valve actuators in thermostatic radiator valves, and in industrial applications, they can use heat to turn on a device.

“Energy harvesting still has massive potential to address the production challenges that we face, beginning with remote-sensing devices and mobile devices,” says Lightman.

Although energy harvesting sensors are a small part of the market, less than one percent, says Bouchard, they offer advantages of reduced maintenance costs, ease and convenience. He adds their use in IoT applications are still in their infancy but it is in new applications that many see the potential.

Innovation will mean steady growth in the number of energy harvesting sensors, says Mahesh Chowdhary, Director, Strategic Platforms and IoT Excellence Center, STMicrolectronics. Energy harvesting can enable self-powered wireless sensors, he predicts. “This approach will allow for greater freedom by eliminating any need for provisioning to power the sensor node throughout the life of the unit,” he observes.

This nascent market is subject to some insecurity, arising from a lack of standardization and a fast-paced end market, where end customer requirements are subject to change as ‘green’ issues, costs and return on investment vie for position. Sounding a note of caution, Wolfgang Schmitt, Senior Manager, Bosch Sensortec Strategic Marketing, advises that while there are savings in terms of service and maintenance time, replacement batteries and waste, “you have to make sure that the corresponding energy harvesting methodology will always work and deliver sufficient energy under all possible conditions.”

For Michael J. Perrotta, CEO, microGen, energy harvesting brings another opportunity for a company to add value to a customer. “It isn’t about what energy harvesting can do, it is about what the company supplying the energy harvesting [technology] can do to help the engineer: time to market, reduced engineering budget, reduced technical risk, extending product life and providing a competitive edge. . . So while energy harvesting is important, more important is understanding the system that needs to be delivered to meet customer needs.”